Raspberry Pi issues


The RPI is a complex controller board; ideal for video streaming multimedia and the like but possibly not so good for the vast majority of simpler applications. The GPIO that is provided is in reality unnecessary as it is not directly accessible to the normal user, it is not buffered and so can be easily damaged. A safer more comprehensive I/O can be provided by a serial, USB or I2C link. Once the external hardware has been added the cost advantage is removed. All of the serial and USB products will work with the PRi but some I2C products do not work due the the none standard RPi I2C interface.


The RPi can only supply a limited current, even on the 5V line so some larger LCD displays will not work on the RPi supply. They can easily be powered from an independent supply.
Independant 5V.jpg
The above example will work for a device that requires 5V at more current than the Pi can supply.


5V equipment usually has 5V logic signalling and so there would be a problem connecting this to the Pi. This is not the case for I2C as the outputs are open collector. What this means is that the voltage will depend on where the pull up resistors are connected to, in the case of the Pi it has them built in and connected to the 3.3V supply. The diagram above therefore will not present any problems.

Where equipment does output 5V then this can be limited with a simple circuit:
5V input.jpg
I have not tested this but see no reason why it should not work. The 100R resistor may be a bit low, try with 1K first to reduce the power needed by the 5V output circuit. For input to 5V from the Pi most 5V logic will work with 3V3. The thing to do is try it, if its not reliable then you know where the issue is.